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8640b Power Supply Repair


8640b Power Supply Repair

Early in the repair cycle of the 8640b, when I had just finished the brass gearset installation, I ran into a problem during the first power up after reinstallation of the deviation and range module. When I powered up the unit, I saw a flash from under the main board, and the unit came up with no led frequency display. I suspected that I had lost one of the power supplies. After finding the manual for the generator, I saw that there were 3 power supply boards with led indicators on them to indicate the various voltages were being generated.

5v indicator

I found that the 5v indicator was not lit. Looking at the schematic of the board, I tracked down that it should have some relatively high voltage DC coming in on pins 15 and 9 of the module ( or pins 1 & 2 of the rectifier module). It had nothing coming in. I saw in the schematic that it was fed from a rectifier board. Since this had easy access, I pulled the board and checked all of the diodes with my ohmmeter. All checked good.

5 volt power supply input

Next, I checked for AC coming in to the input of the rectifier board on pins 19 and 20. Again, there was none. That left only the transformer as a likely suspect. I should have started with a visual inspection, but for some reason I had just skipped that step (shame on me!)

When I turned the unit over, I noticed that the solder joints for the transformer pins had burned circuit board material around some, and had some holes in the solder, indicating bad solder joints. There were also clear indications that this section had been repaired before. This is where the flash I had seen had come from.

Bad solder joints

A closer look shows how ugly it was…

Bad solder joints

I decided to unsolder all of the transformer connections, pull the transformer and add more length to pins as necessary, clean the circuit board and resolder the transformer into place. I also sucked the solder away from the pins of the connector next to the transformer and resoldered that connector, as well.

After all of that work, I wasn’t able to remove all of the charred circuit board, but it looks like I got good connections on my new solder joints. All is working well with the power supply now.

Resoldered connections


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